1. Field of the Invention
The present invention relates to a substrate for a light emitting device, a light emitting device, and a process for the production of a light emitting device. Particularly, the present invention relates to a light emitting device which is used for various devices such as a displaying device, an indication devices and a back lighting device for a liquid crystal display, a substrate used for the production of such light emitting device, and a production process for such light emitting device. In the light emitting device as described above, various light emitting mechanisms (such as an electroluminescent (which may be referred to as xe2x80x9cELxe2x80x9d) mechanism, a photoluminescent (which may be referred to as xe2x80x9cPLxe2x80x9d) mechanism, and a light emission mechanism by means of electron radiation) may be used. In addition, the present invention relates to a plane lamp (such as a flat fluorescent lamp) or a plasma display panel in which such light emitting device is used.
2. Description of the Related Art
Hitherto, various displaying devices have been developed with progress of the intelligent society. One of those devices is an EL device (or an electroluminescent device) which is expected to be used for an electronic display of a self-emission type. The EL device makes use of a luminous phenomenon which occurs upon application of an electric field to a material, and has a structure in which an inorganic EL layer or a organic EL layer is sandwiched by electrodes.
FIG. 11 shows a basic structure of one example of such an organic EL device wherein a transparent electrode 12 as an anode which is made of indium-tin oxide (ITO), an organic EL layer 13 and an back metal electrode 14 as a cathode are laminated in the referred order on a glass plate 11. In such a device, a hole injected from the transparent electrode 12 and an electron injected from the back metal electrode 14 recombines in the organic EL layer 13, whereby an emission center such as a fluorescent dye is excited to result in the light emission. The light emitted in the organic EL layer 13 ejects from the glass plate 11 directly or after being reflected by the back metal electrode 14 made of for example aluminum.
Upon the ejection of the light as described above, an external efficiency (xcex7, which is also referred to as a coupling-out efficiency) which is defined by a ratio of a light quantity drawn outside the light emitting device to a light quantity generated inside the light emitting device is determined by the critical angle xcex8c when the light is totally reflected upon its ejection from a medium of which refractive index is n to the ambient air of which refractive index is 1.0 according to a theory of the classical optics. According to the laws of refraction, the critical angle xcex8c is given by the following equation (1):
sin xcex8c=1/nxe2x80x83xe2x80x83(1)
The external efficiency (xcex7) is obtained by the following equation (2) based on a ratio of a quantity of light passing into the ambient air from the medium of which refractive index is n to a quantity of the generated light (i.e. sum of a quantity of light totally reflected at an interface between the air and the medium and a quantity of light passing into the air):
xcex7=1xe2x88x92(n2xe2x88x921)1/2/nxe2x80x83xe2x80x83(2)
It is noted that when the refractive index n of the medium is greater than 1.5, the following approximate equation (3) may be used:
xcex7=1/(2n2)xe2x80x83xe2x80x83(3)
However, when the refractive index n of the medium is close to 1.00, the above equation (2) has to be used.
Since a thickness of the organic EL layer 13 and a thickness of the transparent electrode 12 are smaller than a wavelength of the light, a refractive index of the glass plate 11 mainly contributes to the external efficiency (xcex7). The refractive index of the glass is generally in the range between about 1.5 and 1.6, so that the external efficiency (xcex7) is about 0.2 (20%) according to the equation (3). The balance which is about 80% is lost as guided light by means of the total reflection between the glass plate 11 and the ambient air.
In the above, an example in which the inorganic or organic EL layer is used as a light emitting member has been explained, and the same explanation is applicable to a PL device in which a PL light emitting layer 15 is used as a light emitting member. FIG. 12 shows a basic structure of the PL device in which a PL light emitting layer 15 is laminated on a glass plate 11. With the PL device, when a ray such as an ultraviolet ray is irradiated onto the PL layer 15, the PL layer 15 generates light, which ejects from the glass plate 11. For the PL device, the external efficiency (xcex7) is small as in the case of the above EL device, and much light is lost as the guided light.
As described above, the external efficiency upon drawing the light generated in the EL device or the PL device from the device into the ambient air (i.e. a coupling-out efficiency for surface emission) is small, and such a small external efficiency is a problem not only to the EL device or the PL device but also to a general problem throughout a light emitting device which ejects surface plane-form light generated in the device into the ambient air.
The present invention has been made considering the above problem, and an object of the present invention is to provide a light emitting device of which external efficiency to draw light outside is higher and of which surface luminance is higher, a substrate for such light emitting device and a process for the production of such light emitting device.
In the first aspect, the present invention provides a substrate for a light emitting device which substrate comprises an electrically conductive transparent film (electrically conductive and transparent film) which is in contact with at least one surface of a low refractive index member. The substrate leads to a higher external efficiency of light which passes through the low refractive index member into the air, so that using such substrate makes it possible to produce a light emitting device of which external efficiency is higher to draw the light into the outside of the device.
In the first aspect, the substrate for the light emitting device in the first embodiment is characterized in that it includes the electrically conductive transparent film which is in contact with the at least one surface of the low refractive index member of which refractive index is greater than 1 and not greater than 1.30. This substrate leads to a particularly higher external efficiency of light which passes through the low refractive index member into the air, so that using such substrate results in an effective light emitting device of which external efficiency is higher to draw the light into the outside of the device.
For example, the low refractive index member may be for example in the form of a layer, a sheet or a plate, and has the electrically conductive transparent film on one of two surfaces which define the low refractive index member. The low refractive index member may be considerably thicker in the above described form, and in such case, the low refractive index member may have substantially at least three surfaces, and for example it may be in the form of a rectangular parallelepiped, in which the low refractive index member may have two or more surfaces which have the electrically conductive transparent electrodes respectively. In the first embodiment of the first aspect, the refractive index of the low refractive index member is in the range preferably between 1.003 and 1.300 and more preferably between 1.01 and 1.2.
In the first aspect, the substrate for the light emitting device in the second embodiment is characterized in that the low refractive index member in the first embodiment of the first aspect is made of an aerogel. The aerogel may be any known aerogel as far as its refractive index is small as described above. In the substrate, when the aerogel is used, it is advantageous in that the low refractive index member which has a further lower refractive index in the above specified range is obtained.
In the first aspect, the substrate for the light emitting device in the third embodiment is characterized in that the low refractive index member in the first or second embodiment of the first aspect is made of a silica aerogel. The silica aerogel may be any known silica aerogel as far as it has the low refractive index. When the silica aerogel is used for the low refractive index member, it is advantageous in that the low refractive index member has excellent transparency and that it is inorganic so that it is stable upon the preparation of the electrically conductive transparent film.
In the first aspect, the substrate for the light emitting device in the fourth embodiment is characterized in that the low refractive index member in any one of the first to the third embodiments of the first aspect has two surfaces which are opposed to each other, and the electrically conductive transparent film is positioned on one of those surfaces and a transparent member is positioned on the other surface. That is, the electrically conductive transparent film is placed on a surface of one side of the low refractive index member while the transparent member is placed on a surface of the other side of the low refractive index member. Placing the transparent member as described above results in the protection of the low refractive index member by means of the transparent member. In addition, since the transparent member supports the low refractive index member, the strength of the substrate as a whole is conferred. As a result, the low refractive index member can be formed in a smaller thickness.
In the first aspect, the substrate for the light emitting device in the fifth embodiment is characterized in that the electrically conductive transparent film in any one of the first to the fourth embodiments of the first aspect is made of at least one material selected from the group consisting of indium-tin oxide (ITO), indium-zinc oxide, zinc-aluminum oxide, gold, silver, copper and chromium. The electrically conductive transparent film is particularly preferably made of indium-tin oxide (ITO). Since those materials make possible to form the electrically conductive transparent film having a higher transparency, the substrate for the light emitting device according to the present invention is preferable for the light emitting device.
In the first aspect, the substrate for the light emitting device in the sixth embodiment is characterized in that the transparent member in any one of the first to the fifth embodiments of the first aspect is made of a glass or a transparent resin (or a plastic material). The transparent member of the glass or the transparent resin may be for example in the form of a layer, a sheet or a plate, or it may be a considerably thicker form. Forming the transparent member of the glass or the transparent resin makes the transparent member itself strong, so that the strength of the substrate is ensured. In addition, the transparent member protects the low refractive index member.
In the first aspect, the substrate for the light emitting device in the seventh embodiment is characterized in that the low refractive index member such as one made of the silica aerogel in any one of the first to the six embodiments of the first aspect is in the form of a thin film. In this embodiment, when the low refractive index member is in the form of the thin film made of the aerogel such as the silica aerogel, spin-coating or dip-coating on the transparent member followed by supercritical drying can easily form the aerogel such as the silica aerogel in the form of the thin film.
In the first aspect, the substrate for the light emitting device in the eighth embodiment is characterized in that the low refractive index member such as one made of the silica aerogel in any one of the first to the seventh embodiments of the first aspect has been made hydrophobic. When the low refractive index member has been made hydrophobic, degradation of performances of the aerogel such as the silica aerogel as to for example the refractive index and the light transparency is prevented.
In the second aspect, the present invention provides a light emitting device which comprises a luminous layer and the substrate for the light emitting device according to the first aspect, and particularly according to any one of the first to the eighth embodiments of the first aspect. This light emitting device uses the substrate for the light emitting device which provides the higher light external efficiency so that a larger quantity of the light generated in the luminous layer is drawn into the outside (i.e. into the ambient (or surrounding) air) of the light emitting device.
In the second aspect, the light emitting device in the first embodiment of the second aspect is characterized in that it comprises the luminous layer and the substrate for the light emitting device according to the first aspect, and particularly according to any one of the first to the eighth embodiments of the first aspect, and that the electrically conductive transparent film has the luminous layer on its one surface which is opposite to its other surface which has the low refractive index member thereon. That is, this light emitting device has the luminous layer on a surface of the substrate for the light emitting device which surface is opposed to the low refractive index. When such light emitting device is used, upon drawing the light which is generated in the luminous layer into the ambient air, the external efficiency of the light which passes through the low refractive index member into the air is increased.
In the second aspect, the light emitting device in the second embodiment of the second aspect is characterized in that the luminous layer in the first embodiment of the second aspect is an organic EL layer. This light emitting device has a higher external efficiency of the light generated in the luminous layer, passing through the low refractive index member and withdrawn into the ambient air.
In the second aspect, the light emitting device in the third embodiment of the second aspect is characterized in that the luminous layer in the first embodiment of the second aspect is an inorganic EL layer. This light emitting device has a higher external efficiency of the light generated in the luminous layer, passing through the low refractive index member and withdrawn into the ambient air.
In the third aspect, the present invention provides another light emitting device which comprises a luminous layer which is in contact with a low refractive index member. This light emitting device has a higher external efficiency of the light which passes from the luminous layer, through the low refractive index member and into the air is increased.
In the third aspect, the light emitting device in the first embodiment of the third aspect is characterized in that it comprises the luminous layer which is in contact with at least one surface of the low refractive index member of which refractive index is greater than 1 and not greater than 1.30. With this light emitting device, a ratio of a quantity of light which passes through the low refractive index member and ejects into the ambient air to a quantity of light which generates in the luminous layer, that is the light external efficiency is particularly increased.
In the third aspect, the light emitting device in the second embodiment of the third aspect is characterized in that the low refractive index member in the first embodiment of the third aspect is in the form of a thin film and is located on a transparent member, and the luminous layer is located on the low refractive index member in the form of the thin film.
In the third aspect, the light emitting device in the third embodiment of the third aspect is characterized in that the low refractive index member in the first or the second embodiment of the third aspect is made of an aerogel, and preferably of a silica aerogel.
In the third aspect, the light emitting device in the fourth embodiment of the third aspect is characterized in that the transparent member in the second or the third embodiment of the third aspect is a plate and preferably a glass plate.
In the third aspect, as to the low refractive index member having the refractive index greater than 1 and not greater than 1.30, the low refractive index member being in the form of the thin film, the low refractive index member being made of the aerogel and preferably made of the silica aerogel, and the transparent member being in the form of the plate and preferably in the form of the glass plate, it is to be noted that the explanations described above in conjunction with the same matters in the first aspect are applicable.
Therefore, in the third aspect, one example of the light emitting device is a plane light emitting device in which the luminous layer (such as a PL luminous layer) is formed above the glass plate, and the silica aerogel in the form of the thin film is formed between the glass plate and the luminous layer (such as a PL luminous layer). With such a plane light emitting device, the light which generates in the luminous layer passes through the silica aerogel thin film having the lower refractive index and goes into the glass plate, so that a ratio of a quantity of the light which is lost as the guided wave is reduced and the external efficiency of the light which is withdrawn from the surface of the glass plate (i.e. the coupling-out efficiency for surface emission) is increased, whereby the luminance of the glass plate surface is increased.
In the fourth aspect, the present invention provides a further light emitting device characterized in that it comprises a luminous layer located on a transparent member, and the luminous layer is made of a low refractive index member which contains particles of a luminescent material dispersed therein or which carries such particles. With this light emitting device, the luminous layer is made of a material for the low refractive index material and the luminescent material in the form of the particles, and such luminous layer having the particles and formed in the light emitting device is produced by forming a coating of a slurry which contains the particles and a binder followed by firing the coating so as to attach the luminous layer to the transparent member strongly. This aspect is advantageous in that the material for the low refractive index functions as the binder, so that the external efficiency of the light is increased
In the fourth aspect, the light emitting device in the first embodiment of the fourth aspect is characterized in that the low refractive index itself (i.e. the material for the low refractive index) has a refractive index which is greater than 1 and not greater than 1.3, and that the luminous layer is located in contact with at least one surface of the transparent member.
In the fourth aspect, the light emitting device in the second embodiment of the fourth aspect is characterized in that the low refractive index in the first embodiment of the fourth aspect is made of an aerogel and preferably of a silica aerogel.
In the fourth aspect, the light emitting device in the third embodiment of the fourth aspect is characterized in that the transparent member in the first or the second embodiment of the fourth aspect is in the form of a plate and preferably in the form of a glass plate. In this embodiment, the luminous layer is preferably located on one of two surfaces which defines the transparent member and which are opposed with each other.
In the fourth aspect, as to the low refractive index member having the refractive index greater than 1 and not greater than 1.30, the low refractive index member being in the form of the thin film, the low refractive index member being made of the aerogel and preferably made of the silica aerogel, and the transparent member being in the form of the plate and preferably in the form of the glass plate, it is to be noted that the explanations described above in conjunction with the same matters in the first aspect are applicable.
Therefore, in the fourth aspect, one example of the light emitting device is a plane light emitting device in which the luminous layer is made of the thin film of the silica aerogel which contains the particles of the luminescent material (such as a PL luminescent material) dispersed therein or which carries such particles. With such a plane light emitting device, the light which generates in the particles of the luminescent material passes through the silica aerogel around the particles having the lower refractive index and goes into the glass plate, so that a ratio of a quantity of the light which is lost as the guided wave is reduced and the external efficiency of the light which is withdrawn from the surface of the glass plate is increased, whereby the luminance of the glass plate surface is increased.
The luminous layer of the light emitting device in any one of the embodiments of the third and the fourth aspects may be a PL luminous layer or a layer which emits light by means of irradiation of an electron for an electron beam). In such light emitting device, since the luminous layer emits light by means of the photoluminescence or the electron irradiation, an electrically conductive film is not necessarily formed on a surface of the low refractive index member, and it is advantageous in that even when the electrically conductive film is formed, a light emitting performance of the light emitting device is not so affected by performances of the electrically conductive film.
In the fifth aspect, the present invention provides a process for the production of a light emitting device, and particularly the light emitting device according to the fourth embodiment of the third aspect. This process is to produce a light emitting device comprising a silica aerogel thin film on a glass plate and a luminous layer (such as a layer made of a PL luminescent material) on the silica aerogel thin film, which process is characterized in that an alkoxysilane solution is applied on the glass plate followed by gelation of the solution to form a gel material and drying the gel material whereby the silica aerogel thin film is formed, and then the luminous layer is formed on the silica aerogel thin film. With this process, the silica aerogel thin film is easily formed so that the production of the light emitting device such as a plane light emitting device becomes easy.
In the sixth aspect, the present invention provides a process for the production of a light emitting device, and particularly the light emitting device according to the third embodiment of the fourth aspect. This process is to produce a light emitting device comprising a luminous layer on a glass plate which layer is made of a low refractive index member in the form of a thin film which contains particles of the luminescent material dispersed therein or which carries such particles, which process is characterized in that an alkoxysilane solution which contains the particles dispersed therein is applied on the glass plate followed by gelation of the solution to form a gel material and drying the gel material whereby the luminous layer in is formed as the thin film which is made of the silica aerogel which contains the particles of the luminescent material dispersed therein or which carries such particles. With this process, the silica aerogel thin film which functions as the luminous layer is easily formed so that the production of the light emitting device such as a plane light emitting device becomes easy.
In the seventh aspect, the present invention provides a plane lamp (for example a flat fluorescent lamp) comprising a plane light emitting device characterized in that the light emitting device of the fourth embodiment of the third aspect or the third embodiment of the fourth aspect is used as the plane light emitting device. With this plane lamp, since a luminous plane is formed by means of the plane light emitting device as described above, and the luminance of a surface of the plane light emitting device is large, a bright plane lamp is produced.
In the eighth aspect, the present invention provides a plasma display comprising a plane light emitting device characterized in that the light emitting device of the fourth embodiment of the third aspect or the third embodiment of the fourth aspect is used as the plane light emitting device. With this plasma display, since a luminous plane is formed by means of the plane light emitting device as described above, and the luminance of a surface of the plane light emitting device is large, a bright plasma display is produced.
It is to be noted that the light emitting device according to the present invention may be flat or curved as a whole. That is, the forms of the substrate for the light emitting device and the luminous layer which form the light emitting device may be flat or curved. Thus, when the light emitting device is of a flat form, each of the low refractive index member, the electrically conductive transparent film and the transparent member is flat. When the light emitting device is of a curved form, each of the low refractive index member, the electrically conductive transparent film and the transparent member is curved with substantially the same curvature. When the flat light emitting device is used, a flat plane lamp or a flat plasma display is produced. When the curved light emitting device is used, a curved plane lamp or a curved plasma display is produced.
In any one of the embodiments of any one of the aspect as described above, a material for the low refractive index member may be an aerogel, and particularly a silica aerogel. The form of the low refractive index member may be a layer, a sheet or a plate of which thickness dimension is considerably smaller than the other dimensions, or the form of the low refractive index member may have a thickness dimension which is of the same order as the other dimensions. An appropriate form of the low refractive index member may be selected depending on the application of the substrate for the light emitting device or the light emitting device itself.
In addition, in any one of the embodiments of any one of the aspect as described above, a material for the transparent member is a transparent glass or a transparent resin. The form of the transparent member may be a layer, a sheet or a plate of which thickness dimension is considerably smaller than the other dimensions, or the form of the low refractive index member may have a thickness dimension which is of the same order as the other dimensions. An appropriate form of the transparent member and an appropriate material for the transparent member may be selected depending on the application of the substrate for the light emitting device or the light emitting device itself.
Further, in any one of the embodiments of any one of the aspect as described above, the electrically conductive transparent film is made of at least one material selected from the group consisting of indium-tin oxide (ITO), indium-zinc oxide, zinc-aluminum oxide, gold, silver, copper and chromium. The form of the electrically conductive transparent film is a layer and particularly a thin layer. An appropriate thickness of the electrically conductive transparent film and an appropriate material for the electrically conductive transparent film may be selected depending on the application of the substrate for the light emitting device or the light emitting device itself.